Process of producing cemented chromium carbide using phosphorous



3,532,492 PROCESS OF PRODUCING CEMENTED CHRO- MIUM CARBIDE USING PHOSPHOROUS William A. Powell, Detroit, Mich., assignor to General Electric Company, a corporation of New York No Drawing. Original application May 1, 1968, Ser. No. 725,929. Divided and this application Nov. 8, 1968, Ser. No. 794,447

Int. Cl. C22c 29/00 US. Cl. 75-204 3 Claims ABSTRACT OF THE DISCLOSURE Cemented carbide alloys containing chromium carbide and a nickel binder are prepared from chromium carbide and nickel powder to which has been added from a trace to about 0.4% by weight of phosphorus. The addition of phosphorus to the cemented carbide composition has been found to considerably enhance the resulting transverse rupture strength of the alloy without compromise of the remaining desirable properties of the alloy, including high hardness, general corrosion and oxidation resistance and high thermal cofficient of expansion.

This is a division of application S.N. 725,929, filed May 1, 1968, now Pat. No. 3,445,203.

This invention relates to a process for preparing cemented chromium carbide alloys having considerably enhanced strength.

Cemented carbides are well known for their unique combination of hardness, strength and abrasion resistance and are accordingly extensively used in industry as cutting tools, drawing dies and wear parts. Presently, the most extensively used cemented carbide alloys are com posed of tungsten carbide and cobalt because of their unexcelled combination of hardness and strength or toughness. However, the tungsten carbide-cobalt alloys have certain deficiencies, such as their relatively low oxidation resistance and their suspectibility to corrosion in media such as aqueous acids. Moreover, they have a very low coefiicient of thermal expansion relative to that of steel. Thus, for example, the high hardness of the tungsten carbide-cobalt alloys makes them attractive materials for gauge blocks but their thermal expansion characteristics differ appreciably from that of steel, the material against which the gauges are most frequently applied. Tungsten-containing alloys are also of high density and cobalt-containing alloys are generally magnetic, characteristics which are a disadvantage in certain applications.

Other alloy systems, such as titanium carbide and tantalum carbide, have similar shortcomings. Improved corrosion resistance can be achieved by using nickel and nickel alloys as a binder in tungsten carbide alloys, but such alloys still have relatively poor oxidation resistance, display comparatively low thermal expansion characteristics and are of relatively high density.

Chromium carbide-nickel cemented carbide alloys possess a number of very desirable properties. They are characterized by a higher coefiicient of thermal expansion than the other cemented carbidescloser to that of steel a low density, they are nonmagnetic, and they are generally more resistant to oxidation and to corrosion in 3,532,492 Patented 00L 6, 1970 aqueous acid media. They possess high hardness, like the other cemented carbide alloys. However, the chromium carbide-nickel alloy systems heretofore available have displayed a very serious deficiency relative to the other cemented carbides that has considerably restricted their usage. They are significantly lower than the other cemented carbide systems in strength or specifically transverse rupture strength.

Accordingly, it is an object of this invention to provide a cemented carbide alloy of high hardness, having good corrosion and oxidation resistance, comparatively low density, comparatively high coeflicient of thermal expansion, which is nonmagnetic and which, in addition, possesses high strength. More specifically, it is an object of this invention to provide a chromium carbide-nickel base alloy combining high hardness with high strength. It is an additional object of this invention to provide a process for preparing such an alloy.

I have found that the foregoing and other objects of this invention may be achieved by the addition of certain small but critical amounts of phosphorus to the composition from which the cemented carbide alloy is prepared. More specifically, the addition of amounts ranging from a trace to about 0.4% by weight, based on the total weight of the composition, of phosphorus to a chromium carbide-nickel powder mixture produces a cemented carbide alloy having a strength which is enhanced as much as 100% over a comparable chromium carbide-nickel composition prepared without the phosphorus addition. The alloys of the invention are prepared by pressing a mixture of the aforementioned powders and sintering the pressed mixture into a compact. The phosphorus is preferably added to the starting mixture in the form of a nickel-phosphorus alloy in amounts adjusted so as to contain the necessary phosphorus quantity.

The compositions of this invention contain chromium carbide essentially in the form of Cr C and a binder of nickel, the binder ranging from about 1, and preferably 3, to about 35% by weight. Other metals may be added to the nickel binder and specifically up to one-third of the nickel may be replaced by molybdenum or tungsten.

The present compositions have very excellent resistance to oxidation relative to that of cemented carbide alloys in general. They also display good resistance to corrosion in media such as aqueous acids. They have low density, approximately one-half that of tungsten carbide-cobalt alloys. They are nonmagnetic unlike the cobalt-containing alloys. They have a high coefiicient of thermal expansion, more nearly that of steel or about double that of tungsten carbide, titanium carbide and tantalum carbide base alloys. They are of high hardness. Unlike the chromium carbide-nickel base alloys heretofore available, the compositions of this invention are of high strength, having up to approximately twice the strength of the chromium carbide-nickel base alloys heretofore available.

Whereas the prior chromium carbide base compositions, being low in strength, were necessarily adjusted compositionally so as to maximize strength, the compositions of the present invention are sufficiently strong such that compromises may be made between strength and hardness. For example, the tungsten carbide-cobalt alloys vary in tungsten carbide content from about to 97% by weight, and in cobalt content from about 25% to about 3% by weight. Maximum strength is achieved in alloys of high cobalt content. Maximum hardness is achieved in alloys of high tungsten carbide content but some sacrifice in strength must be made toaccomplish the latter. The improvements provided by the present invention are such that similar compromises between strength and hardness can now be made within useful limits in chromium carbide base alloys. Thus, the compositions of the present invention may vary in chromium carbide content from about 65% to about 99% by weight. Strength is greatest in alloys of high nickel content, being about double that of the chromium carbide base alloys heretofore available. Hardness is greatest in alloys of high chromium carbide content, and these have hardness values as high as about 91 R -considerably higher than that of existing chromium carbide commercial alloys, which generally run(s) from 8789 R even though their strengths are equal.

The addition of phosphorus in the very small amounts herein disclosed is believed to alter the metallurgical phenomena occurring during sintering as well as the microstructure of the resulting alloys. While the true explanation of this effect is not known with certainty, it is believed that the presence of phosphorus improves bonding between the carbide grains and the nickel matrix and results in some modification in the microstructure of the alloy.

The desired properties are achieved only within the approximate compositional limits indicated above. Alloys containing more than about nickel, or other nickelbase binder such as nickel-molybdenum or nickel-tungsten, do not have the requisite hardness, whereas alloys containing less than 1% nickel are low in strength. If more than about 0.4% phosphorus by weight is employed, the hardness of the resulting alloys suffers substantially. It is difiicult to define in absolute terms the exact minimum amount of phosphorus that is required. The strength improvement has, however, been achieved with extremely small amounts. On the one hand, it is of course necessary that a definite amount of phosphorus be present. On the other hand, the few parts per million that may be present in starting materials of usual purity is not sufficient. Because of inconsistencies and problems associated with the dispersion of very small amounts, additions of a trace amount of the order of 02% phosphorus (about .12% nickel-phosphorus alloy of 17% phosphorus content) has been found to be an approximate minimum.

The carbon content of the starting chromium carbide Cr C should normally be between 13.0% and 13.3% by weight, preferably between 13.2% and 13.3%. It is believed that the carbon of the chromium carbide powder employed herein is essentially all combined. Again, this is not known with certainty because the analysis of carbon in chromium carbide is very diflicult and the results subject to some doubt, even utilizing the best analytical techniques available.

The process of the invention is carried out by first intimately mixing the chromium carbide powder, nickel metal powder and a source of phosphorus, the latter prefarably being in the form of a mixture of a transition metal such as nickel and phosphorus. A nickel-phosphorus alloy may be made, for example, by reacting ammonium phosphates with nickel metal powders at elevated temperatures, and subsequently crushing and pulverizing the solidified melt. Phosphorus may also be added directly as ammonium phosphate or as an anhydride of phosphorus but the oxygen present in combined form must be carefully expelled by reduction or dissociation prior to sintering. The above-mentioned powders are intimately mixed by ball-milling in a liquid medium such as acetone. The milled powder is then dried in a protective atmosphere, and a small amount of paraffin is added to facilitate pressing of the powder. The pressed compacts are heated to about 450 C. in a protective atmosphere to expel the paraffin. The compacts are then sintered in a protective atmosphere at about 1200l300 C.

The following examples illustrate the preparation of compositions in accordance with the present invention. All parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1 An alloy containing 17% nickel, 83% chromium carbide, and about .04% phosphorus was prepared as follows:

Commercially available chromium carbide powder of 13.1% carbon was adjusted to the desired carbon level in 10 kilo lots. For the purpose, 86 grams of chromium powder were added as a carburization promoter together with 33 grams of carbon (lamp black) such that the carbon content of the total mix was about 13.3%. These powders were intimately mixed and heated in carbon boats in a hydrogen atmosphere at a temperature of 1400 to 1500 C. for a period of approximately one hour. The resulting mass was crushed and pulverized to 325 mesh powder.

A nickel-phosphorus alloy was prepared by reacting diammonium phosphate and nickel powder in quantities sufficient to provide a 17% phosphorus alloy. For this purpose, the mixture was melted in carbon boats in a hydrogen atmosphere at a temperature of approximately 1250 C. The cooled mass was quite friable and easily converted to 200 mesh powder.

Two hundred and forty-nine grams of the above-mentioned chromium carbide powder, 1 gram of the 17% nickel-phosphorus alloy, and 51 grams of nickel powder were ball-milled in a 4" cemented tungsten carbide lined ball mill containing 250 grams each of A", and /s cemented tungsten carbide balls, with 200 cc.s of acetone for 24 hours. The powder was then dried in a hydrogen atmosphere and 7 /2 grams of paraffin were added as a pressing lubricant. The powder was pressed into the desired shape at 15 t.s.i. pressure and the compacts were presintered in a hydrogen atmosphere at 450 C. The presintered compacts were then placed in a vacuum furnace and heated at 1250 C. for 15 minutes. The resulting pieces had a hardness of 88.5 R a density of 7.03 gm./cc., and a transverse rupture strength of 250,000 p.s.i. This is approximately twice the strength of heretofore available chromium carbide-nickel alloys.

EXAMPLE 2 An alloy containing 8% nickel, 92% chromium carbide and about .04% phosphorus was prepared as described above, from 276 grams of chromium carbide powder, 24 grams of nickel powder, and one gram of the nickel-phosphorus alloy. In this case, the sintering temperature was 1275 C. This alloy had a hardness of 90.3 R a density of 6.87 gm./cc., and a strength of about 200,000 p.s.i., over 50% greater than that (about 120,000 p.s.i.) of heretofore available alloys.

EXAMPLE 3 An alloy containing 83% chromium carbide, 13.6% nickel, 3.4% molybdenum and about .04% phosphorus was processed as indicated in Example 1, from 249 grams of chromium carbide powder, 4008- grams of nickel powder, 10.2 grams of molybdenum powder, and about one gram of nickel-phosphorus alloy. The sintering temperature in this case was 1275 C. This alloy had a hardness of 89.2 R a density of 7.1 gm./cc., and a strength of about 210,000-220,000 p.s.i., about 70% more than the heretofore available alloys.

I claim:

1. A process for producing a cemented carbide alloy comprising pressing a mixture of chromium carbide and nickel binder powder containing from a trace up to 0.4% by weight of phosphorus and sintering said pressed mixture into a compact.

6 2. The process of claim 1 in which the phosphorus is OTHER REFERENCES added to the mixture as an alloy with the nickel binder. Cemented carbides, Schwarzkopf & Kiefier 3. The process of claim 1 in which the nickel binder The MacMinan ca 1960 pp 177 181 is present in percentages ranging from 335% by weight 0f the total composition 5 CARL D. QUARFORTH, Primary Examiner Referen es Cited A. J. STEINER, Assistant Examiner UNITED STATES PATENTS US CL 3,245,763 4/1966 Fall 29 1s2.7 7s 201, 214

3,390,967 7/1968 Frehn 29-482] 10 

